This invention relates generally to the field of radio frequency power amplifiers. More particularly, certain embodiments of this invention relate to a distributed RF power amplifier having compensation for varying loads such as impedance variations due to antenna location in a portable hand-held product.
Portable hand-held devices such as telephones and two-way radios are subject to operation in various environments that can dramatically affect the immediate surroundings of the device""s antenna. For example, a cellular telephone might be carried in a purse, held near the user""s head, clipped on the user""s belt or might simply be resting on a desk or car seat. In each case, the wide variations of antenna surroundings affects not only the antenna""s radiation pattern, but also the antenna""s characteristic impedance as seen by the device""s radio frequency (RF) power amplifier (PA). Such impedance variations can and do seriously impact the transmission of power from the power amplifier to the antenna. As the antenna and the power amplifier become mismatched as a result of the antenna""s surroundings, standing waves are created and power is reflected from the antenna back into the power amplifier. The result is that the power amplifier can become unstable, operate at significantly reduced efficiency, perform outside of design parameters or even draw excessive DC current which may result in damage to the power amplifier.
This problem has been addressed by use of an RF isolator circuit situated between the power amplifier and the antenna and/or by over-design of the RF power amplifier to protect it from unacceptable load conditions. The use of an isolator, however, can substantially impact the usable bandwidth of the power amplifier. Both solutions can add substantial expense to the transmitter circuitry.